1. Field of the Invention
This invention relates to radio frequency communications and, more particularly, to a frequency division duplex/time division duplex (FDD/TDD) system and method for radio frequency communication for greater quantities of information over a limited spectrum for lower cost by providing a secondary communications channel, in addition to a primary communications channel, for full duplex communication over both the primary and secondary channels.
2. Brief Description of Related Technologies
Radio components can be some of the most expensive portions of radio frequency (RF) communications equipment. This is particularly the case in cordless, or wireless, telephony. In RF communications and particularly, in cordless or wireless telephony, costs and operational requirements determine the viability of communications equipment designs. The engineering designer is often presented with design constraints imposed by the costs of component, or by operational requirements. Such costs and operational requirements are particularly important considerations when communications equipment is intended for lower-end consumers, such as individuals and households.
Cordless phones based on analog and/or continuous variable slope delta (CVSD) digital methods use FDD to provide a wireless link between a base and handset. Since FDD requires two frequencies to accommodate a full-duplex link, the two carriers are used 100% of the time during a call. To adequately support two handsets an FDD base station design would require one RF transceiver for each handset. Since the cost of a cordless phone depends on not only the handset cost but the base station cost as well, the cost of the system increases.
Products which advertise similar capability either mean a two-line (i.e. PSTN) to two handset capability, provide a second RF transceiver at the base, or are designed to allow either handset to detect an incoming call but only one to be used during a call; the second handset is then unable to share in the conversation.
In RF communication technology, including cordless phones, various standards established by industry and other sources often dictate performance and equipment requirements. Standards have been established, for example, for cordless or wireless telephony products and other communications devices. Some examples of the most common standards of the cordless telephone industry include: the Cordless Telephony Second Generation (CT2) standard, the European Conference of Postal and Telecommunications Administrations (CEPT) standard, referred to as the Cordless Telephony First Generation (CT1) standard, the Cordless Telephony First Generation Plus (CT1+) standard, and the Digital European Cordless Telecommunications (DECT) standard.
The CT2 standard, for example, employs a time division duplex (TDD) system and methodology. In TDD, transmit and receive communications occur among two stations, such as, for example, a handset and base set unit of a cordless telephone, in a burst manner at distinct intervals of time. In the past, devices conforming to CT2 have transmitted and received over an identical carrier frequency within the bandwidth dictated by the standard. Communications have been possible in TDD units because different time intervals are employed for transmissions and receptions by each station. During an interval that one station is transmitting, the other is receiving, and vice versa, over the same bandwidth. Devices built according to the CT2 standard have been considered more spectrally efficient than FDD based devices. This is due, in part, attributable to the use in those devices of only a single radio channel for transmitting and receiving. However, the primary way to transmit and receive more data in a CT2 system was to compress the data transmitted in the allotted time interval. This is disadvantageous since as data is further compressed the devices become more expensive by requiring more processing capabilities and system bandwidth, which causes communications quality to sulfer.
Other cordless telephone standards, such as the CT1 standard, have at times employed a frequency division duplex (FDD) concept. In typical FDD, transmit and receive communications occur over two distinct, separate carrier channels. Thus, two FDD communications stations, such as, for example, a handset unit and a base set unit of a cordless telephone, each transmit and receive over different carrier channels. While a first unit is transmitting over a particular channel at a first frequency, the second unit is receiving on that same channel over the same frequency. When the second unit transmits, it does so on a separate channel at a second frequency and the first unit receives or that different channel at that second frequency. FDD systems tend to be less expensive than TDD systems because of their analog nature of conveying voice signals over a communications channel when compared to the digital nature of TDD based systems.
Beyond TDD and FDD, other communications methodologies are continually being developed. Certain promising methodologies include use of a dual channel synthesizer and spread spectrum approaches. Common FDD based designs use a dual synthesizer approach for synthesizing the necessary channels for communication. TDD designs, on the other hand, require the generation of another oscillator source for accomplishing communications over a single carrier frequency. However, the spectrum efficiency of such a system is less than for a TDD based system, since two Frequencies are utilized instead of one. But, the additional filtering needed for reducing spurs in a TDD system makes such a system less attractive from a cost viewpoint. A drawback of this approach is that additional filters are required to attenuate the frequency image (referred to herein as "Spurs"). The use of dual frequency sources, as used in A FDD implementation, alleviates that requirement at the expense of using a second synthesizer and local oscillator.
In spread spectrum communications technology, a sequential noise-like signal structure is employed to spread normally narrow band information signals over a relatively wide band of frequencies. The receiver in such systems correlates the special spectrum transmission signals to retrieve the original information being transmitted. Spread spectrum technology provides certain benefits, such as greater information over one channel. A disadvantage of spread spectrum technology, however, is increased expense because significant processing capacity is necessary to transmit and receive over the broad spectrum of frequencies employed.
As is apparent, there are numerous ideas and approaches to radio frequency communications in cordless telephony. On the one hand, TDD methods can be advantageous because of the minimal spectrum necessary for such communications. On the other hand, FDD methods provide advantages of continuous and simultaneous transmission and reception and lower system cost. The limitations of both such methods, though, are apparent, as previously discussed. The newer communications methods also provide certain advantages, but they do so only at greater expense or by use of more frequency spectrum. It would, therefore, be a significant improvement in the art and technology to provide an improved system and method for radio frequency communication which is low cost, and yet allows for communications of larger amounts of continuous and simultaneous information, such as voice and data information.
The present invention employs a unique system and method for a hybrid digital FDD/TDD radio frequency communications format in which a secondary channel is provided to allow radio frequency communications between two mobile units and a base unit over the channels to occur concurrently. FIGS. 1a and 1b illustrate the FDD and TDD schemes of the prior art.
Though the background of the invention has been described, in part, with respect to cordless telephone applications and, in particular, to applications under the various standards and emerging technologies described above, the invention has other varied applications which will be hereinafter more fully understood.